Stacked metal inductor

ABSTRACT

An inductor has a conductor layer formed by multiple concentric co-planar turns of ultra-thick metal (UTM) adapted to receive current at a frequency of at least one gigahertz. The multiple turns of UTM proceed from an innermost turn to an outermost turn, and aluminum stacking is provided over all of the UTM turns except at least the innermost turn, thereby optimizing the Q of the inductor.

BACKGROUND

The present disclosure relates generally to inductors of the type havingmultiple planar windings composed of a layer of ultra-thick metal (UTM)with an aluminum (AL) layer stacked thereon. More particularly, thepresent disclosure concerns an inductor of this type which has anoptimize quality factor (Q), preferably a higher Q.

The present disclosure also concerns a method for making such aninductor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing results of a simulated integrated circuit DCO(Digital Controlled Oscillator) design with varying inductor Q formetallic conductors of different resistances.

FIG. 2 is a graph showing the relationship between inductor Q andinductor metal resistance in a further simulation.

FIG. 3 is a perspective view of a three turn inductor with aluminum (AL)stacking over UTM.

FIG. 4 shows L and Q comparison graphs of two simulated TXDCO8inductors, one with stacking AL over UTM, and the other with UTM only.

FIG. 5 shows L and Q comparison graphs of two simulated TXDCO3inductors, one with stacking AL over UTM, and the other with UTM only.

FIG. 6 shows the proximity effect with respect to respective turns of afour turn stacked inductor.

FIG. 7A is a perspective view of a four turn inductor in accordance withone aspect of the disclosure, with no stacked aluminum layer on theinnermost turn.

FIG. 7B shows graphs of L and Q comparison of TXDO8 inductors, for UTMonly for all turns, UTM with AL stacking for all turns, and UTM with noAL stacking on the innermost turn.

FIG. 8A is a perspective view of three turn inductor in an aspect of thepresent disclosure, with no stacked AL on the innermost turn.

FIG. 8B is a perspective view of a three turn inductor in accordancewith the disclosure, with no stacked AL on the two innermost turns.

FIG. 8C shows graphs showing L and Q comparisons of TXDCO3 inductors,with UTM only for all turns, UTM with AL stacking for all turns, UTMwith no AL stacking on the innermost turn, and UTM with no AL stackingon the two inner turns.

DETAILED DESCRIPTION

An inductor is one of the fundamental passive components, together withresistors and capacitors that are used in a wide variety of electricalcircuits. The classic definition of a conductor is a passivetwo-terminal component that resists changes in electric current passingthrough it. An inductor is basically composed of a conductor, such as awire, that is usually wound into the form of a coil, either as a coil(three-dimensional) or a spiral (planar configuration). Energy is storedin the magnetic field produced in the coil as long as current is flowingthrough the coil. When current flowing through the coil changes, thetime varying magnetic field resulting therefrom induces a voltage in theconductor according to Faraday's law of electromagnetic induction. Therelationship between the time-varying voltage v(t) across an inductorhaving an inductance L, and the time-varying current i(t) passingthrough the inductor is described by the differential equation:v(t)=Ldi(t)/dt.

When the current through the inductor is an alternating (AC) current,the amplitude of the voltage is proportional to the product of theamplitude of the current and the frequency of the current.

Inductors are commonly found in almost all wireless radio-frequencyintegrated circuits that are currently in use. One example is aninductor that is used in a voltage controlled oscillator (VCO) ordigital controlled oscillator (DCO). A DCO or VCO is composed of threebasic components, namely an inductor L, a capacitor C, and an activetransistor circuit that provides the energy (signal source). The DCO orVCO will oscillate at a frequency F based on the relationshipf=1/(2π√LC).

The DCO/VCO precision low noise radio-frequency signals, or clockreferences, are considered to be the heart of modern wireless integratedcircuits. Due to the complexity of today's communication integratedcircuits for cell phone applications, there are often multiple VCO/DCOsin a single transceiver integrated circuit, generating various clocks atdifferent frequencies.

One of the most important parameters in designing an inductor is itsquality factor, or Q. The Q of an inductor is defined as (2πfL)/R, wheref is the frequency of the current flowing through the inductor, L is theinductance, and R is the resistance at the operating frequency. Ingeneral, the higher the Q, the better the inductor, because a high Qtranslates to lower phase noise for the clock signal generated using theinductor, and a lower current drain due to its lower ohmic loss. Resultsof simulation of an integrated circuit DCO design operating at four GHzwith varying Q by a few percent are shown in the table below:

% increase inductor metal resistance Increase in DCO current (μA) 1 59 2118 3 177 4 236 5 295

FIG. 1 shows a decreasing phase noise for lower inductor metalresistance for the designated different resistances. The impact ofinductor Q on current drain or power consumption is more direct becausethe inductor resistance to energy dissipated at steady state oscillationis:P _(diss) =I _(bias) ² R _(s),wherein I_(bias) is the bias current, and R_(s) is the inductor's seriesresistance.

FIG. 2 is a graph showing the DCO current drain, wherein it can be seenthat an almost linear relationship exists with regard to the total DCOresistance, which is the LC tank resistance plus the active deviceresistance. The calculation of the current change with the changinginductor metal resistance is summarized in the table above, indicatingthat a 5% increase of inductor resistance corresponds to an increase incurrent of 295 μA.

Lower current drain is the most critical factor in today's competitivemobile communication market. Due to the importance of low current drain,significant effort has been given to improving Q of the inductor used insuch circuits. A way to obtain such an improvement would be to increasethe width of the windings of the inductor. Increasing the width,however, would lower the inductance L, and in order to maintain the sameinductance L, the inductor size or area would have to increasesubstantially in order to compensate, which is a huge penalty. Octagonalor circular shape inductors are often used instead of rectangularinductors in critical designs, due to their slightly higher Qperformance. Some approaches make use of a patterned ground shield (PGS)and slotting the inductor windings in order to improve Q. When designingan RF/analog (CMOS) wafer process platform, the primary inductorwindings typically reside in the available ultra-thick metal (UTM)layer, usually a copper layer that is used to due to its low sheetresistance. An aluminum layer is then applied above the UTM, or thethinner copper layers below are used for crossovers. In order to furtherreduce the resistance R of the inductor, one practice is to copy UTMinductor windings to the aluminum layer, and to stack the aluminum layeron top of the UTM, except at the crossovers. The stacked aluminum andUTM layers are then stitched together with an array of vias in an effortto thicken the inductor cross section, thereby lowering its resistanceR. Such an integrated inductor 1 composed of UTM windings 2 with an ALlayer 3 stacked on top thereof is shown in FIG. 3. For clarity, notevery via 4 is shown in FIG. 3. The shown vias 4 exist in every branchof every turn of the inductor in FIG. 3.

An insight on which the present disclosure is based is that at GHzfrequencies, or tens of GHz frequencies, where DCO/VCO inductorsoperate, the high-frequency current flowing in one part of the inductorcan be altered by the H field generated from another part of theinductor. This is a known phenomenon called the proximity effect. In thecase of an inductor of the type shown in FIG. 3, when the aluminum layeris stacked on the UTM layer, the proximity effect can push more currentinto the higher-resistive aluminum layer, causing a higher R (andtherefore lowering Q) than that of an inductor using only a UTM layer.The present disclosure is based on a study that shows for multi-turninductors, the inductor's inner turns (windings) suffer more proximityeffect than the inductor's outer turns, due to the H field distributionin that type of inductor. The present disclosure is based on the insightof optimizing the Q of a multi-turn inductor by stacking the aluminumlayer over the UTM layer on only the outer turns of the inductor, whileleaving one or more inner turns uncovered by the aluminum layer, andthus composed of UTM only. This structure is easy to implement, andleads to a significant Q improvement.

The aforementioned solution can be easily generalized in terms ofinductor design, so that no elaborate inductor patterns are involved,and the solution can be easily applied to existing inductors with nomajor re-design and with minimum layout change. The technique inaccordance with the present disclosure can be combined with knowntechniques, such as PGS. The technique in accordance with the presentdisclosure can be applied to inductors having an octagon shape, slottedinductors, or inductors having the shape of a figure eight foradditional Q improvement. The technique can also be used in all knownCMOS RF/analog process with different metal stacks. Because the strengthof the proximity effect depends on many factors, including the relativethickness of the UTM and aluminum layers (sheet resistance ratio ofaluminum to UTM), the optimum point to begin stacking the aluminum layeron the inductor outer turns may be different for various processes, butthe principal applies to all wafer processes and all inductor designs.

In order to provide a basis for evaluating the Q improvement obtained inaccordance with the present disclosure, two inductors (TXDCO3 andTXDCO8) were simulated, in order to characterize their L and Q,comparing the option of using UTM only and the option of using stackedaluminum over UTM. For the four turn TXDCO8, the UTM-only option showeda higher Q than that of the AL+UTM option, as shown in FIG. 4. This canbe explained by the proximity effect, in which the current in onewinding is affected by the H-field generated by its neighboringwindings, and is thereby pushed into the higher resistive aluminumlayer, thereby causing a higher effective R (and Q becomes lower), thanan inductor without an aluminum layer, at frequencies above 2 GHz.However, for the three-turn TXDCO3, the Q of the UTM-only inductorappears to be approximately the same as that of the UTM+AL inductor, asshown in FIG. 5, indicating that the proximity effect appears to be notlarge enough to offset the lowering of R with stacking aluminum.

Further studies show that the proximity effect and the inductor Rinteract in a more complex way, which can be illustrated in the exampleof a four-turn inductor as follows. The four-turn inductor 5 can bedecomposed into four progressively smaller concentric one-turn coils, asshown in FIG. 6. From electromagnetic theory, it is known that each coilgenerates an H-field from the current in that coil, but the H-field isconcentrated only in inside of the coil, and is diminished outside ofthe coil. This means that the outermost coil does not experience theproximity effect, because the H-field generated by all of the innercoils is contained within the inner coils, and very little of theH-field proceeds through the windings of the outermost coil at alocation at which current is flowing therein. The second outermost coilthen suffers from the proximity effect only by the H-field of theoutermost coil, etc., until the innermost coil is reached. The innermostcoil will suffer the most proximity effect, because that coilaccumulates H-fields from all of the other outer coils. Consequently,for optimizing the Q of a multi-turn inductor, the inductor inaccordance with the present disclosure uses AL+UTM layers on the outerturns, where the proximity effect is weak in order to reduce R, but usesUTM only on the inner turns, in order to avoid the proximity effect.

One aspect of the disclosure is shown in FIG. 7A, which is a simulatedTXDCO8 inductor 6 with the aluminum layer of the innermost turn 7removed. The same simulation as described above for the inductor wasre-run, which the results being shown in FIG. 7B for comparison of L andQ with the previous AL+UTM and all UTM-only scenarios. For the inductorin accordance with the present disclosure, the Q was improved byapproximately 5% over UTM only or AL+UTM.

FIG. 8A shows a TXDCO3 inductor 8 in accordance with the disclosure(three-turn inductor 8 as opposed to four-turn inductor 6 shown in FIG.7A), with the aluminum layer removed from the innermost turn 7. FIG. 8Bshows a TXDCO3 inductor 9 in accordance with the present disclosure withthe aluminum layer removed from the two inner layers 7 and 10. FIG. 8Cshows the simulation results for these aspects of the disclosure. Thesesimulation results show that UTM-only for all turns has the lowest Q andadding stacked AL would improve Q, but the highest Q (approximately 5%better than all UTM-only or all AL+UTM) is achieved by stacking AL onthe outer two turns, and removing AL on the innermost turn. Removing thealuminum layer on additional turns begins degrading Q again, and hencean optimum Q exists for multi-turn inductor designs wherein a fixednumber of inner turns uses UTM only, and all outer turns use AL+UTM.This number may be one.

The impact of the proximity effect on the inductor Q may depend onfactors including the sheet resistance ratio between the aluminum layerand the copper UTM layer (processed dependent), the ratio of inductorwidth/spacing/turns (design dependent), as well as other factors, but Qcan be optimized in accordance with the present disclosure by stackingaluminum over only the outer turns of the inductor, while leaving asingle layer of UTM only on the inner turns. To determine the cutoffpoint of using or not using stacked aluminum for optimum Q, eitherelectromagnetic simulation can be used, or test chips can be easilymeasured. Stacking or not stacking an aluminum layer on a particularturn can shift the inductance L slightly, but this shift is small(approximately 2%) and can be taken into account in the overall inductordesign, absorbed into PVT, or compensated with capacitance. Animprovement in Q of 5% in accordance with the inductor design of thepresent disclosure translates to an estimated 295 μA of current drainsaving.

Although the above examples are described in the context of inventors,the described techniques can also be used for improving Q in devicescomposed of inductors/coils that are wound together, such as baluns ortransformers.

Example 1 is an inductor comprising a conductor layer comprising aplurality of concentric co-planar turns of ultra-thick metal (UTM)adapted to receive current at a frequency of at least one gigahertz,said plurality of turns of UTM proceeding from an innermost turn to anoutermost turn, and aluminum (AL) stacking over all of said UTM turnsexcept at least said innermost UTM turn.

Example 2 is the inductor of Example 1 comprising aluminum stacking overall of said UMT turns except said innermost UTM turn and anext-to-innermost UTM turn.

Example 3 is the inductor of Example 1 wherein only said outermost UTMturn has aluminum stacking thereon.

Example 4 is the inductor of Example 1 wherein said UTM turns and saidaluminum stacking are octagonal in shape.

Example 5 is the inductor of Example 1 wherein said UTM comprisescopper.

Example 6 is the inductor of Example 1 consisting of four of said UTMturns, with aluminum stacking over all of said UMT turns except theinnermost turn of said four UTM turns.

Example 7 is the inductor of Example 1 consisting of three of said UTMturns, with aluminum stacking over all of said UMT turns except theinnermost turn of said three UTM turns.

Example 8 is the inductor of Example 1 consisting of three of said UTMturns, with aluminum stacking over all of said UMT turns except theinnermost turn of said three UTM turns and a next-to-innermost turn ofsaid three UTM turns.

Example 9 is a conductor layer comprising at least three concentricco-planar turns of ultra-thick metal (UTM) adapted to receive current ata frequency of at least one gigahertz, said at least three turns of UTMbeing arranged outwardly from an innermost turn to an outermost turn andaluminum (AL) stacking over all of said UTM turns except said innermostUTM turn and a next-to-innermost UTM turn.

Example 10 is the conductor layer of Example 9 wherein said UTM turnsand said aluminum stacking are octagonal in shape.

Example 11 is the conductor layer of Example 9 wherein said UTMcomprises copper.

Example 12 is a method for manufacturing an inductor, comprising forminga conductor layer to have a plurality of concentric co-planar turns ofultra-thick metal (UTM) adapted to receive current at a frequency of atleast one gigahertz, said plurality of turns of UTM being arrangedoutwardly from an innermost turn to an outermost turn, and aluminumstacking over all of said UTM turns except at least said innermost UTMturn.

Example 13 is the method of Example 12 further comprising providingaluminum stacking over all of said UMT turns except said innermost UTMturn and a next-to innermost UTM turn.

Example 14 is the method of Example 12 further comprising providing onlysaid outermost UTM turn with aluminum stacking thereon.

Example 15 is the method of Example 12 further comprising forming saidUTM turns and said aluminum stacking to be octagonal in shape.

Example 16 is the method of Example 12 further comprising using copperas said UTM.

Example 17 is the method of Example 12 further comprising forming onlyfour of said UTM turns, with aluminum stacking over all of said four UMTturns except the innermost turn of said four UTM turns.

Example 18 is the method of Example 12 further comprising forming onlythree of said UTM turns, with aluminum stacking over all of said UMTturns except the innermost turn of said three UTM turns.

Example 19 is the method of Example 12 further comprising forming onlythree of said UTM turns, with aluminum stacking over all of said UMTturns except the innermost of said three UTM turns and anext-to-innermost of said three UTM turns.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the Applicant to embody within thepatent warranted hereon all changes and modification as reasonably andproperly come within the scope of the Applicant's contribution to theart.

The invention claimed is:
 1. An inductor, comprising: a conductor layercomprising a plurality of concentric co-planar turns of ultra-thickmetal (UTM) adapted to receive current at a frequency of at least onegigahertz, the plurality of turns of UTM proceeding from an innermostturn to an outermost turn; and an aluminum (AL) stacking layer formeddirectly over and making electrical contact with all of the UTM turnsexcept for (i) at least the innermost UTM turn, and (ii) one or morecrossovers between each of the plurality of turns of UTM.
 2. Theinductor as claimed in claim 1, wherein the aluminum stacking layer isformed directly over all of the UTM turns except for the innermost UTMturn and a next-to-innermost UTM turn.
 3. The inductor as claimed inclaim 1, wherein only the outermost UTM turn has the aluminum stackinglayer thereon.
 4. The inductor as claimed in claim 1, wherein the UTMturns and the aluminum stacking layer are octagonal in shape.
 5. Theinductor as claimed in claim 1, wherein the UTM comprises copper.
 6. Theinductor as claimed in claim 1, wherein the plurality of concentricco-planar turns is four UTM turns, with the aluminum stacking layer overall of the UTM turns except the innermost turn of the four UTM turns. 7.The inductor as claimed in claim 1, wherein the plurality of concentricco-planar turns is three UTM turns, with the aluminum stacking layerover all of the UTM turns except the innermost turn of the three UTMturns.
 8. The inductor as claimed in claim 1 wherein the plurality ofconcentric co-planar turns is three UTM turns, with the aluminumstacking layer over all of the UTM turns except the innermost turn ofthe three UTM turns and a next-to-innermost turn of the three UTM turns.9. The inductor of claim 1, wherein the absence of the aluminum stackinglayer over the innermost UTM turn results in a reduction in a proximityeffect as compared to all of the UTM turns having the aluminum stackinglayer thereon, the proximity effect being caused by a magnetic fieldgenerated from the plurality of concentric co-planar turns loops otherthan the innermost loop.
 10. The inductor of claim 1, wherein theinductor exclusively includes the conductor layer and the AL stackinglayer.
 11. An inductor comprising: a conductor layer comprising at leastthree concentric co-planar turns of ultra-thick metal (UTM) adapted toreceive current at a frequency of at least one gigahertz, the at leastthree turns of UTM proceeding from an innermost turn to an outermostturn; and an aluminum (AL) stacking layer formed directly over andmaking electrical contact with all of the UTM turns except (i) theinnermost UTM turn, (ii) a next-to-innermost UTM turn, and (iii) one ormore crossovers between each of the at least three concentric co-planarturns of UTM.
 12. The inductor as claimed in claim 11, wherein the UTMturns and the aluminum stacking layer are octagonal in shape.
 13. Theinductor as claimed in claim 11, wherein the UTM comprises copper.
 14. Amethod for manufacturing an inductor, comprising: forming a conductorlayer to have a plurality of concentric co-planar turns of ultra-thickmetal (UTM) adapted to receive current at a frequency of at least onegigahertz, the plurality of turns of UTM proceeding from an innermostturn to an outermost turn; and forming an aluminum stacking layerdirectly over to make electrical contact with all of the UTM turnsexcept (i) at least the innermost UTM turn, and (ii) one or morecrossovers between each of the plurality of concentric co-planar turnsof UTM.
 15. The method as claimed in claim 14, further comprising:providing the aluminum stacking layer over all of the UTM turns exceptthe innermost UTM turn and a next-to innermost UTM turn.
 16. The methodas claimed in claim 14, further comprising: providing only the outermostUTM turn with the aluminum stacking layer thereon.
 17. The method asclaimed in claim 14, further comprising: forming the UTM turns and thealuminum stacking layer to be octagonal in shape.
 18. The method asclaimed in claim 14, further comprising: using copper as the conductorlayer.
 19. The method as claimed in claim 14, further comprising:forming four of the UTM turns, with the aluminum stacking layer formedover all of the UTM turns except the innermost turn of the four UTMturns.
 20. The method as claimed in claim 14, further comprising:forming three of the UTM turns, with the aluminum stacking layer formedover all of the UTM turns except the innermost turn of the three UTMturns.
 21. The method as claimed in claim 14, further comprising:forming three of the UTM turns, with the aluminum stacking layer formedover all of the UTM turns except the innermost of the three UTM turnsand a next-to-innermost turn of the three UTM turns.